If America was populated by 330 million murderers, we wouldn't have a population anymore. Criminals don't obey gun laws. Ban a firearm and there will be a flood of them on the black market, fresh from the wide-open southern border. Prison inmates have been interviewed and the vast majority of respondents said they feared armed, law-abiding citizens far more than law enforcement. Why? Because there are so many more armed citizens that cops; with concealed carry, it could be anyone around them (so they aren't immediately recognizable); and police have rules of engagement, giving criminals a tactical advantage that doesn't exist with citizens. This is why most mass shootings occur in places that restrict gun ownership. Lawful, responsible firearms ownership and possession is a deterrent to violent crime - it's been proven again, and again, and again.
Remember the Colorado shooter who shot people in the movie theater? He drove past five other theaters to go to the one that was marked "gun free zone". Why? Little to no chance of armed resistance. That's why in the more than one thousand school districts, in twenty states, allowing school teachers and staff to discreetly carry personal firearms during school hours, there hasn't been a single shooting - let alone a mass shooting - at any of them. In the early 1970s, Israel suffered a school shooting every week. In 1974 a law was passed allowing staff and plain-clothes officers to carry firearms in school. In the nearly 50 years since, there have been only two shooting attempts... in both cases, the shooter was killed before they could inflict mass casualties.